The present invention related generally to differential measurement probes and more particularly to a differential measurement probe having ground clip system for probing tips that move axially and/or laterally relative to each other.
Differential measurement probe have first and second probing tip extending from a probe body for acquiring differential signals from a device under test or for acquiring a single signal with the second probing tip connected to a ground node of the test device. As the speed of electronic signal increases, inductive and capacitive effects that were negligible at lower bandwidths become increasing important. Inductive ground loops are one effect that can limit the bandwidth of a differential measurement probe. As the bandwidth of differential measurement probes approach 20 GHz, there is a need to reduce inductive ground loops as much as possible.
In a related application, the increasing speeds of electronic signals has resulted in the need for transmission line structures in printed circuit board (PCB) designs. To optimize high performance PCB designs for high speed applications, smooth transmission line structures are need to link communications between components. Time domain reflectometry (TDR) probes launch a rising or falling edge signal onto transmission line structures on a printed circuit board and acquire the return signal from the transmission line structures for determining parameters of the transmission lines. For example, the verifying the integrity of a transmission line structure can be determined using a TDR probe and a sampling oscilloscope.
Although instruments for differential TDR measurements are in existence, the limitations of commercially available TDR probes has resulted in PCB manufacturers having to rely upon test structures laid out on test coupons that are placed along the periphery of the PCB flat for PCB transmission line impedance control measurements. TDR data from the test coupon is used for determining lot quality for accepting or rejecting the circuit boards. This has lead to rejecting marginally good boards and allowing marginally bad boards because the test coupon may be far away from the actual transmission lines of interest. Correlation studies between system speed and transmission line designs are typically based on the test coupon results. Because of the difficulty in controlling the laminate layer thicknesses, the dielectric constant variations, metal line photo edge definitions and the copper etching over large board areas, there is usually sizable variations in the impedance of transmission lines depending on the their board location. The non-uniformity between transmission lines is typically on the order of ten percent. Due to these problems, correlation studies for high performance differential transmission lines to board impedance often requires large quantities of samples to reveal the true relationship.
Another problem a user encounter when performing differential TDR test is the need to provide a good ground for the two differential signal lines. Typically when performing differential signal measurements on a printed circuit board, a coplanar probe pad arrangement is required. The general probe pad arrangements are ground-signal-signal-ground (G-S-S-G) or a ground-signal-ground-signal-ground (G-S-G-S-G). This is a very restrictive requirement because the line spacing and line width of differential pairs are of may different varieties depending on the device pin pitch, PCB board materials, desired loss limit, and the like. A differential TDR probe has to be able to accommodate these different feature sizes.
An example of a TDR probe is the CP400-04, manufactured by Candox System of Japan. The probe has a metal housing in which an insulated signal conductor is disposed. The metal housing has a threaded connector at one end for connecting a signal cable. The other end of the housing has apertures for receiving spring action pogo pins. One pogo pin is coupled to the insulated signal conductor and the other pogo pins are connected to the metal housing. The resulting probing tips have a GSG configuration with 2.5 millimeter center-to-center spacing between the pogo pins.
A further example is the A0131688 TDR Probe, manufactured and sold by Inter-Continental Microwave, Santa Clara, Calif. The TDR probe has a metallic housing with one end of the housing having a threaded connector for connecting a signal cable. A substantially rectangular member extends outward from below the connector and has a threaded aperture for receiving a screw that secures the TDR probe to the flat spring when the TDR probe is configured with a similar probe for differential TDR applications. Below the rectangular member is a circular portion that transitions into a narrow rectangular probe tip member. The probe tip member has an aperture that receives an RF pin and dielectric member. The RF pin is electrically connected to a central signal contact of the treaded connector. Additional apertures are formed in the narrow rectangular probe tip member for receiving ground pogo pins. The various apertures allow the ground pogo pins to be positioned at various distances from the RF pin. The resulting probing tip has a GSG configuration.
Two A0131688 TDR Probes are used to produce the A0134332 Differential TDR probe, manufactured and sold by Inter-Continental Microwave, Santa Clara, Calif. The individual TDR probes that are mounted to a flat spring using two screws. A variable spacing adjustment clamp is position over the TDR probes adjacent to the narrow rectangular probe tip members. The adjustment clamp has a “U” shaped portion and a flat portion with the two portions being secured together with screws. The two opposing sides of the “U” shaped member have threaded apertures that receive adjustment cap screws that extend through the sides of the “U” shaped member and into interior space of the “U”. Treaded apertures are formed in the base of the “U” shaped member that intersect the threaded apertures in the opposing sides of the “U” shaped member. Each threaded aperture in the base receives a set screw that is tightened on the adjustment cap screws.
Positioning of the RF pins are accomplished by loosening the set screws on the adjustment cap screws and turning the adjustment cap screws to move each TDR probes closer together or farther apart. The flat spring to which the TDR probes are attached causes outward pressure on the probes to force them against the adjustment cap screws. The screws holding the TDR probes to the flat spring may also be loosened to allow rotational movement of the probes. When the RF tip and the ground pogo pins are positioned correctly, the set screws and the flat spring screws are tightened.
U.S. Pat. No. 6,734,689 describes a measurement probe providing signal control for an EOS/ESD protection control module. The measurement probe has a spring loaded coaxial probe assembly and a pressure sensor that work in combination to provide an activation signal to the control module. The control module is coupled to a TDR module in a sampling oscilloscope that provides the rising or falling edge signal to the DUT and samples the return signal from the DUT. The spring loaded coaxial cable assembly and pressure sensor are disposed in a probe housing. The spring loaded coaxial probe assembly has a semi-rigid coaxial cable with one end forming a probing tip and the other end having a threaded connector. A flexible coaxial cable is connected to the threaded connector and to the control module. A ground probing tip is disposed adjacent to the probing tip and is electrically coupled to the outer shielding conductor of the semi-rigid coaxial cable. The ground probing tip is a retractable, spring loaded probing tip that is attached to a slotted collar that fits around outer shielding conductor of the semi-rigid coaxial cable. The resulting probe has a GS configuration.
What is needed is a differential measurement probe that reduces inductive ground loops for achieving a 20 GHz probe bandwidth. Further, there is a need for a variable spacing differential TDR probe that is not limited to existing ground-signal-ground configurations. The variable spacing differential TDR probe should be provided with a ground clip system that couples the outer shielding conductors of the coaxial probing tips together during all possible axial and lateral movements of the coaxial probing tips.